1. Field of the Disclosure
The present disclosure is generally directed to drilling fluids that are used during drilling operations for oil and gas wells, and in particular, to methods and apparatuses for analyzing the properties of such drilling fluids.
2. Description of the Related Art
Drilling fluid, or “mud,” is a multicomponent fluid specially formulated to perform various functions during drilling of an oil or gas well. Examples of such functions include cooling the drill bit, lubricating and sealing the wall of the well, providing hydrostatic pressure in the well annulus to prevent formation fluid influx, clearing away drilled solids from the drill bit, and returning drilled solids to the surface. Circulation of the drilling fluid through the well typically involves pumping the drilling fluid down the bore of a drill string in the well, whereby the drilling fluid jets through nozzles in a drill bit at the end of the drill string into the bottom of the well. At the bottom of the well, the drilling fluid commingles with drilled solids and possibly other materials in the well such as gases and returns to the surface with the entrained materials. The return path is typically in an annulus between the wall of the well and the drill string. In dual bore drilling, the return path may be in the drill string. At the surface, a series of actions is taken to rid the drilling fluid of the entrained materials so that the drilling fluid can be pumped into the well again.
Drilling fluid is generally an aqueous or non-aqueous suspension of one or more materials. The suspension medium may be water or brine, oil, or synthetic fluid combined with various chemical additives. The added materials can be minerals, such as barite or bentonite, or synthetic polymers in particulate form. At various stages of removing the entrained materials, it is important to analyze the properties of the drilling fluid in order to ascertain that the drilling fluid has the proper composition and rheology to achieve desired functions. The drilling fluid properties measured are typically density, viscosity, and solids content. Currently, drilling fluid properties are measured using at least four devices, such as a mud balance, a Marsh funnel, a rotating cylinder viscometer, and a mud retort. The measurements are customarily made manually in that a technician collects a sample of drilling fluid, places the sample in the device, reads some type of physical indicator by eye, and records the measurement. These manual measurements are prone to errors that may prove costly in a drilling process.
A mud balance is used to measure drilling fluid density. To measure drilling fluid density, a cup is overfilled with a sample of the drilling fluid while placing a lid over the sample in such a way that there are no bubbles in the volume of drilling fluid remaining in the cup. The cup is then placed at one end of a graduated beam having a bubble level. A slider weight is moved along the length of the beam until the bubble level indicates that the beam is level. The position of the slider weight along the length of the beam indicates the density of the drilling fluid. The accuracy of the density measurement depends on how accurately the technician can read the bubble level and the position of the slider weight. Most mud balance beams are graduated in increments of 0.1 pounds/gallon.
A Marsh funnel is used to measure drilling fluid viscosity. To measure drilling fluid viscosity, the funnel is held vertically while plugging the bottom of the funnel. The drilling fluid is then poured into the funnel through a filter, e.g., a 10 Mesh screen. The filter will prevent solids that can clog the funnel from entering the funnel. When a desired amount of the drilling fluid has been poured into the funnel, the bottom of the funnel is unplugged to allow the drilling fluid to flow into a graduated container. The technician uses a stopwatch to record the time needed for a predetermined volume of the drilling fluid to be released from the funnel into the container. The accuracy of the viscosity measurement depends on the ability of the technician to accurately read the volume of drilling fluid released into the container and to accurately start and stop the stopwatch.
In the Marsh funnel method, filtering of the drilling fluid actually changes the drilling fluid so that the viscosity measured is more an apparent viscosity than a true viscosity. Also, particles in the drilling fluid may segregate inside the funnel while the bottom of the funnel is plugged, which may lead to variations in flow rate out of the funnel that would not be accounted for by merely measuring time and the amount of fluid released into the container. The range of application of the device is limited to those fluids that continue to flow freely as the level inside the funnel drops to very low values. Other fluids not having this property may result in clogging of the funnel by surface tension forces.
A rotating cylinder viscometer is also used to measure drilling fluid viscosity. A typical rotating cylinder viscometer includes two concentric cylinders. The inner cylinder is often referred to as bob and is suspended on a torque measuring device. To measure drilling fluid viscosity, drilling fluid is poured into a chamber through a filter, e.g., a 200 Mesh screen. The filter will prevent solids that can lodge in between the concentric cylinders from entering the chamber. The cylinders are then submerged in the drilling fluid in the chamber, and the outer cylinder is rotated at various rotational speeds while the bob is held stationary. The force necessary to shear the fluid between the cylinders is read from the torque measuring device that holds the bob stationary. Viscosity is estimated using an expression that is a function of the shear torque, the geometry of the cylinders, the angular velocity of the outer cylinder, and the immersion depth of the inner cylinder or bob in the drilling fluid. The rotating cylinder viscometer provides additional value over the Marsh funnel in that multiple rotational speeds correspond to multiple strain rates, which allows non-Newtonian behavior to be recorded for the drilling fluid.
The accuracy of the viscosity measurements made by the rotating cylinder viscometer depends on the accuracy of the rotational speed of the outer cylinder, on the accuracy of the reading of the indicator of the torque measuring device, and on the depth of immersion of the cylinders. In older versions of the device, a hand crank is used to rotate the outer cylinder. The normal procedure calls for turning the hand crank very evenly through a gearbox that allows for several specific speeds, if the handle is turning at a constant rotational speed. However, this constant rotational speed has to be achieved by hand, which can be very difficult. A more modern version of the device uses a motor to drive the outer cylinder at precisely correct rotational speeds, which results in better precision. The viscosity measurements made by the rotating cylinder viscometer are affected by filtering of the drilling fluid received in the chamber, i.e., the actual drilling fluid measured is different from the original drilling fluid that contained the solids removed by the filtering. There is also the possibility of particles settling within the chamber while the viscosity measurements are being made. Particle settling can introduce errors to the measured shear torque.
A mud retort is used to measure many drilling fluid properties, e.g., the density of the drilling fluid, the density of the drilled solids, and the overall solids content of the drilling fluid. To make measurements with the mud retort, a known volume of fluid is heated in a retort chamber, typically to a temperature of over 900° F. Steam and vaporized oil exit the chamber and immediately pass through a condenser that returns the vapors to liquids and delivers the liquids to a graduated cylinder. The volumes of the liquids read from the cylinder are subtracted from the known volume to determine a volume of the solids in the drilling fluid. The mass of the solids can be determined, e.g., on a digital scale. The volume and mass of the solids are used to determine the specific gravity of the solids.
The present disclosure is directed to various methods and apparatuses for measuring the properties of drilling fluids that may address, at least in part, some of the above-described problems associated with prior art devices and methods that are used to evaluate drilling fluid properties.